1. Field of the Invention
The invention relates generally to cellular networks used for performing digital communications over a large geographical area via radio frequency ("RF") links sharing a single assigned channel or frequency. More particularly, the invention relates to methods and apparatus which (1) facilitate decentralizing certain uplink network message control functions and (2) minimizing, or in some cases eliminating, terminal power level assessment overhead typically expended by base stations and network controllers to reliably perform digital communications on RF links using the aforementioned single assigned frequency or channel.
2. Description of the Related Art
Cellular radiotelephone networks are well known that provide communications services over a predefined geographical area divided into zones which are sometimes referred to as "cells". Each cell typically includes a plurality of base stations and associated antennas located at each base station, for transmitting and receiving message signals between a given base station and subscriber radios, sometimes referred to as "terminals", "cellular phones", "radiotelephones", "data radios", and the like, located in or transiting through a given cell.
Each base station in a given cell typically communicates with a general communications controller (a "GCC"), which functions as a centralized control mechanism for coordinating communications between the subscriber radios in a given cell and a host computer, often coupled to or forming part of a telephone switching network.
An example of such a system in commercial use is the Advanced Radio Digital Information System ("ARDIS"), developed jointly by Motorola, Inc. and the International Business Machines Corporation. A specific example of such a system is described in U.S. Pat. Nos. 4,550,443 and 4,850,032, to Freeburg, and in many other technical publications and patents.
In the ARDIS, RF links between the various terminals in a given cell and the base stations within that cell, are dynamically established as the need for communications services and resources arise.
Many of the aforesaid technical publications and patents have addressed the problem of how to support a multiplicity of simultaneous digital communications randomly attempting to make use of the single frequency (also referred to herein as the shared channel) that is usually assigned for a predefined geographical area. This is the case, for example, in U.S. Pat. No. 4,866,788, to Mouly et al, which describes a process for controlling the retransmission of messages from transmitting stations belonging to a cellular system.
The process taught by Mouly et al is based on the recognition that the probability of having to retransmit a request message to use a shared channel depends on (is a function of) the state of the shared channel and the power received by the base station from the transmitting subscriber radios. Subscriber radio transmitting power is measured at the base station and controlled from the base station in order to impact the amount of traffic attempting to access the shared channel at any given point in time.
Mouly et al is one example of how assessing the power level of a signal transmitted by a subscriber radio can be used in performing network control functions. Mouly et al also exemplifies a system in which power level assessment overhead is expended at the base station level of a network hierarchy in order to implement the novel process described in the reference.
Other examples of how power level assessment overhead is expended to support dynamic power level adjustment techniques used in radio telecommunications networks to perform network control functions, to minimize "collisions" with respect to the use of a shared channel, etc., are described in U.S. Pat. Nos. 4,512,033 to Schrock; in 4,613,990, to Halpern; and in the aforementioned 4,550,443 to Freeburg.
Schrock describes circuitry used in a generalized bidirectional communication system, with the circuitry being resident at each of a plurality of remote terminals, for responding to externally generated power level adjustment signals. The externally generated power level adjustment signals are used to control a plurality of remote terminals connected to a master terminal.
Halpern is an example of prior art that teaches performing power level measurement at the fixed base station level of a network hierarchy to dynamically control the power of radiotelephone transmitters.
Neither the Schrock or Halpern references deal with the problem of managing and/or reducing "uplink" message traffic overhead, i.e., overhead associated with message traffic flowing from the remote terminals towards a host computer, particularly in situations where a message transmitted by a single terminal can result in a plurality of uplink messages. This phenomenon occurs whenever a transmitting terminal is within range of two or more of the plurality of base stations typically located within a given cell site.
U.S. Pat. No. 4,550,443, to Freeburg, does present a system that is capable of handling the aforementioned uplink message traffic; however, as will be explained hereinafter, the uplink traffic message overhead expended is considerable and has the potential for having an adverse affect on uplink message throughput.
The Freeburg reference (U.S. Pat. No. 4,550,443) describes a data communications system that covers a geographic area divided into a plurality of cells and includes a general communications controller (GCC), a plurality of channel communications modules (CCMs, or base stations), a set of transmitter and receiver pairs each associated with a given base station, and a plurality of portable terminals. Data signals, included in packets of information that also include control signals, are communicated between the GCC and the portable terminals by way of a radio channel (the data signals are also referred to herein as the "data portion" of a packet). Each base station takes a signal strength measurement every time it receives a packet from a portable terminal.
In systems similar to the one taught in the Freeburg references cited hereinabove, the GCC gathers the signal strength measurements from the base station receivers and all of the packets received by the various base Stations (even duplicate messages received from a given terminal) are passed to the GCC.
The GCC then computes an adjusted signal strength for each input and selects the input having the largest adjusted signal strength for determining the location of the portable radio that transmitted the packet. The GCC sends the packet associated with the strongest signal to the host, after determining that the packet contains valid data as will be explained hereinafter. Additionally, the GCC takes note of the identification (ID) of the terminal/base station pair that provided the strongest signal.
Whenever the GCC thereafter transmits a message to a portable terminal, the base station associated with the recorded terminal/base station pair ID, is the base station of choice to set up the RF link to the terminal. In other words, the base station that covers the area having the largest adjusted signal strength for the last transmission from the target portable terminal is the first choice when trying to communicate "downlink" (i.e., from the host or GCC toward the terminal).
Since the GCC can be simultaneously transmitting message signals to portable radios in other portions of the cell using non-interfering base station transmitters, downlink information throughput is greatly enhanced in systems similar to the one taught by Freeburg.
The above described Freeburg reference (U.S. Pat. No. 4,550,443) is an example of a power level adjustment technique which affects the size of the transmitting "spheres" of base station transmitters. This technique for adjusting the power level of these transmitters facilitates the ability to communicate with more than one terminal in a given cell (in a downlink sense) at any one point in time.
Although the size of the transmitting spheres can be made non-overlapping for downlink communications purposes; uplink communications in a given cell having a plurality of base stations still suffer from the aforementioned centralized overhead problems associated with two or more antennas within a cell picking up signals being transmitted by any given terminal at any point in time.
As a result, not only is equipment required at the base station level of the network hierarchy to determine signal strength for each and every message received from a terminal, the GCC in systems similar to the one described by Freeburg are also forced to expend overhead to sort out which of the received packets generated by a single terminal should be passed on to the host computer. This means that the GCC must determine which of the received packets resulted in the strongest received signal; record the ID of the base station (and the transmitting terminal) that received the signal to provide an indication of the best path to use for downlink communications back to the transmitting terminal; and determine the validity of the packets received from the various base stations before choosing a packet to send on to the host computer.
In order to determine the validity of received packets, cyclic redundancy checking (CRC) techniques, well known to those skilled in the art, are often employed at the base station level of a network hierarchy. A validity indication, like signal strength indication, is placed in the packets that are passed to the GCC which again must expend overhead in making the aforementioned packet validity determinations.
It should be noted that the strongest signal received by a base station and passed to the GCC may contain questionable data. In this case it is not desirable to pass the strongest signal on to the host computer. The GCC would typically throw out the questionable data in favor of a weaker received signal so long as the weaker signal includes a packet that passes, for example, a CRC test, or some other validity test established to verify proper data transfer within the network.
All of the presently known radio frequency data communications networks utilize a GCC or its equivalent to assure that a high quality message is passed on to the host computer, and to record downlink control information to be used in transmitting information from the host or GCC back to a specific terminal. It can be readily appreciated by those skilled in the art that where even a single message sent by a given terminal results in the GCC (1) having to interpret the signal strength of a set of received packets; and (2) having to determine the highest quality signal from among the set of packets, etc., the aforestated GCC overhead problem becomes quickly compounded when many messages are being transmitted by a given terminal, and/or messages are being simultaneously transmitted by a plurality of terminals within a given cell.
Accordingly, it would be desirable to decentralize and minimize the amount of GCC overhead expended in interpreting signal strength measurements, controlling uplink communications, and determining the quality of a set of packets received from a plurality of base stations, where the set of packets relate to the transmission of a single message by a given terminal. As indicated hereinbefore, the desirability of being able to decentralize and minimize GCC overhead for these purposes becomes even more acute in practice where multiple copies of a plurality of messages are all being input to a GCC by the base stations located in a given cell.
It would also be desirable to minimize or even eliminate the need for power level (signal strength) measurement equipment at the base station level in the network hierarchy. As indicated hereinbefore, such equipment is presently being used for the purpose of achieving reliable uplink communications and enabling the GCC to be able to select an appropriate terminal/base station pair for downlink communications.
In fact, it would be desirable to provide a radio frequency data communications network that includes methods and apparatus for determining the optimal terminal/base station pair at the terminal level of the network hierarchy based on signal strength. This is particularly true since, as those skilled in the art will readily appreciate, the terminal side of the terminal/base station link is the "weakest" link in the network hierarchy. Factors such as the terminals often being mobile (the base stations are usually fixed); terminal power supplies being typically less reliable than the power supplies used by the base stations, etc., mitigate in favor of a system in which the terminal picks the best RF link it can establish. In such a network, the terminal chosen RF link would be the link of choice passed on to the GCC for use in effecting downlink communications; rather than making the choice the other way around, i.e., at the GCC level, based on parameters that can quickly become obsolete.
Thus, it would be desirable if each terminal in the network dynamically selected a target base station and included the selected terminal/base station pair identification information within each transmitted packet. This would enable the GCC to easily identify the first choice base station to use to forward packets of data being sent from the host computer (or GCC itself) to a particular terminal, without having to expend any overhead to identify the optimal path to the terminal.
Furthermore, it would be desirable to provide a network, and related methods and apparatus, that operatively assures that only a single valid packet is sent to the GCC by only one of the base stations in a given cell, (for uplink transactions), even where a plurality of base stations each receive a packet being transmitted by a given terminal. This would significantly reduce GCC overhead since the GCC, whenever a packet is received from a base station, would simply have to store the terminal/base station ID and would then simply pass the already validated packet to the host computer.